The present invention relates to a safety circuit for a hydraulic system.
Agricultural tractors are increasingly being equipped with controlled hydraulic systems in which oil is supplied via axial-piston control pumps. For energy considerations, load-sensing systems are used for regulation of the pump. In such systems, the instantaneous delivery stream and pressure of the pump are regulated as a function of the required output of the consuming device. Such hydraulic systems are known as closed-center, load-sensing, hydraulic systems.
In order for the power steering of a hydraulic tractor to be operable at all times, the hydraulic system is provided with a priority valve which is a pressure balance or governor integrated into the pressurized fluid line leading from the control pump to the load hydraulic circuit. The steering valve for the power steering hydraulic circuit, however, is connected directly to the control pump. The pressure difference which occurs at the steering valve acts on a controller piston of the priority valve in such a manner that increased demand for power on the part of the steering will cause the flow of fluid to the load-hydraulic circuit to be throttled.
The axial flow piston control pump used in the above-described hydraulic system has the disadvantage that, regardless of its specific structural development, it reacts relatively sensitively to excessive vacuum in its suction line. One approach to solve this problem has been the provision of an additional feed pump which conveys hydraulic fluid from a storage container into the suction line of the control pump. This maintains a sufficiently high pressure level. However, since the feed pump is generally a gear pump which pumps a constant volumetric flow, it must be designed for the maximum delivery of the control pump. This therefore leads to an uneconomical design. Since the maximum delivery of the control pump is required only infrequently, that is only for periods of time which constitute a mere fraction in the range of 1% of the total operating time of the hydraulic system, the excess amount of the flow conveyed by the feed pump must be sprayed off into the storage container.
Another method has been proposed which uses a feed pump to convey only a fraction of the requirementof the control pump. In case there is a greater need for fluid, the control pump draws fluid, via an additional line, directly from the storage container. However, this approach gives satisfactory results only if the fluid is of sufficient temperature and if the suction line is of a suitable cross section. Otherwise, an unacceptably high pressure drop may occur in the suction line and thus cavitation may occur at the control pump. More particularly, upon a cold start and under low ambient temperature conditions, it is difficult to reliably prevent cavitation because the viscosity of the fluid causes the pressure drop to increase.
One approach to a more optimal hydraulic system may be found in German Patent DE-PS No. 34 45 516, which corresponds to U.S. patent application Ser. No. 807,983 filed Dec 12, 1985. This describes a safety circuit wherein if the pressure drops below a minimum but still permissible suction pressure of the control pump, the pump output will be reduced while the suction pressure is increased. This safety circuit operates reliably for protecting the control pump. However, the component parts of this safety circuit are used exclusively for protection of the control pump. Therefore, additional safety functions, such as insuring that hydraulics are supplied to the power steering hydraulic circuit, cannot be fulfilled by these parts.
Accordingly, it is an object of the present invention to create a safety circuit which reliably protects the control pump from cavitation while being simultaneously used to supply the power steering itself with a hydraulic fluid, even in the event that a leak should occur in the load-hydraulic circuit.
Further it is an object of the present invention to provide a safety circuit for a hydraulic system, comprising a control pump; a priority valve connected to the control pump and being operable in two conditions wherein in one condition the priority valve is held in a throttling position for supplying hydraulic fluid to a plurality of hydraulic loads, and in a further condition is for supplying hydraulic fluid to a single hydraulic load, to the exclusion of all remaining hydraulic loads; and a control circuit having a safety valve which is connected to the control pump for limiting the intake pressure of the control pump, the safety valve being controlled by the intake vacuum of the control pump so as to reduce the output of the control pump in response to a drop in the intake pressure of the control pump below a predetermined value, the safety valve also being connected to the priority valve for maintaining the priority valve in the first operational condition when the intake pressure of the control pump is at or above a predetermined value and for maintaining the priority valve in the second operational condition when the intake pressure falls below a predetermined value.
It has been discovered that, by coupling the priority and safety valves, an advantage is provided which uses the safety valve as a sole, yet doubly controllable control valve should the suction pressure assume an unacceptable level and in the event that the level of liquid in the supply container drops below a predetermined level.
The safety valve therefore provides two functions and results in decreased cost. The reason why the safety circuit can be constructed in the described manner, is that the power steering hydraulic circuit requires relatively little oil as compared with the output capacity of normally installed control pumps. Therefore, when the pressure drops below a minimum specified suction pressure, sufficient reduction of the control pump will always take place thereby permitting the suction pressure to once again rise. Also, an alternate embodiment of the present invention provides a manner in which the already existing parts of the control connection lines can be used as a differentially activated control line for priority valve purposes.
In a preferred embodiment the priority valve is designed as a pressure balance or pressure balance valve. According to the invention i.e. by connecting an input of said priority valve to the hydraulic fluid supply line of said first hydraulic load and the output of said priority valve to said second hydraulic load and by providing a control line transmitting a hydraulic pressure which is representative of the hydraulic pressure requirements of said hydraulic loads it becomes possible to maintain an already proposed design of the hydraulic circuit for the supply of the servo-steering hydraulic load circuit with priority and to give to this circuit at the same time by an especial integration of the pump pressure line new functions wherefrom economical advantages result. Additionally the control of the priority valve can be performed independent from any additional energy supply which results in higher reliability. By the inventive control of the priority valve the latter is maintained by use of a constant spring force in a position which blocks the load hydraulic circuit so that the priority valve upon switching of said safety valve can be shifted to a stable blocking position. If a change-over valve is provided which receives pressures representing the requirements of said hydraulic loads and further having an output having the respective higher hydraulic pressure acting at the respective inputs, wherein said output of said change-over valve is connected to said control valve which controls said output of said control pump as a function of said hydraulic pressure of said output of said change-over valve additional advantages which respect to the design of the control for the control valve result. This is due to the fact, that the control line always leads a pressure which represents the requirement of the respective consumer, so that the control can be performed by a fixed circuit connection.
If a sensor is provided and an actuator device is comprised of a solenoid, whereby said sensor produces an output signal at output terminals thereat only when a fluid level in an hydraulic fluid storage container is above a predetermined level, and wherein output terminals are connected to said solenoid, whereby said output signal maintains said actuator device in a first position which thereby maintains said safety valve in said first switch position, and whereby said solenoid further includes a spring biasing means for urging said solenoid into a further position and thereby urging said safety valve into said second switch position in the absence of said output signal, the level of safety is raised. In case said the control for the solenoid fails the spring biasing means ensure that the servo steering circuit is always supplied with priority.